During the production of electronic devices, there is sometimes a need to mount an integrated circuit die or chip onto a carrier. After mounting, electrical connections are made between the chip and the carrier, typically by the use of bonding wires. The product is then molded into a semiconductor package. These bonding wires may be attached to bond pads on the chip and the carrier respectively by ultrasonic welding using a transducer that is caused to vibrate at ultrasonic frequencies.
A conventional transducer comprises a piezoelectric element for generating ultrasonic vibrations, an ultrasonic amplifying horn and a bonding tool which is normally in the form of a capillary or wedge. The bonding tool is located at the free end of the amplifying horn and is used to apply the bonding force to the components being welded together.
The amplifying horn is usually provided with a flange or other form of mounting member, often known as a barrel or collar, that permits the transducer to be mounted to a bond head of a wire bonding machine for movement in the X, Y and Z axes (and also in some designs about a rotational or theta axis) as required in a sequence of bonding operations.
When the ultrasonic transducer is integrated into a wire-bonding machine, since very fine wires and bond pitches are involved, the manner in which the ultrasonic transducer is mounted to the wire-bonding machine is very important. For example, if the ultrasonic transducer is mounted such that vibrations from the transducer are transferred to a mounting bracket, ultrasonic energy loss can result. If there is excessive vibration, the transducer characteristics, such as its frequency spectrum characteristics, may be changed, thereby affecting the stability of the transducer. Furthermore, the impedance of the transducer may change unpredictably, making it harder to control. Therefore, it is important to mount the ultrasonic transducer such that vibration generated during ultrasonic bonding is isolated to the transducer itself, and as far as possible, these vibrations are not transmitted to the rest of the wire-bonding machine.
One way of mounting the transducer to the bonding machine is by the introduction of a flange. Another way could be to clamp the transducer to the bonding machine.
In the case of a flange-mounted ultrasonic transducer, one way in which vibration to the wire-bonding machine can be minimized is disclosed in U.S. Pat. No. 6,135,339 for “Ultrasonic Transducer with a Flange for Mounting on an Ultrasonic Welding Device, in particular on a Wire Bonder”. This patent teaches locating the flanges on a nodal point of the longitudinal vibration of the horn, and also forming the flange with a sufficient length such as to introduce a nodal point for radial vibration of the horn so as to locate a mounting point at the said radial nodal point. The ultrasonic transducer is mounted at the positions of these radial nodal points so that vibrations at the mounting positions are the lowest possible, and thereby reducing the transmission of vibration to the wire-bonding machine to which the transducer is mounted. There is only one pair of flanges located at the same vibrational nodal position of the horn.
Another example of a flange-mounted transducer is disclosed in U.S. Pat. No. 6,766,936 entitled “Transducer and Bonding Apparatus Using the Same”. It describes two pairs of flange supports located on both sides of an amplifying horn, comprising holding portions that protrude from at least two locations that correspond to the nodes of vibration of the transducer main body, and connecting portions that connect the tip ends of the protruding holding portions to each other.
In the aforesaid patent, the holding portions are located at adjacent nodes of vibration of the transducer main body, which is equivalent to half a wavelength of an oscillatory waveform. However, a distance of the front-side holding portion to a tip of the transducer is more than one wavelength away. The problem is that the suspended part of the amplifying horn from the front-side holding portion and the tip of the transducer is quite long, resulting in less rigidity in the suspended portion. Hence, the rigidity of this part of the horn must be increased by the use of harder materials or increasing the cross-section of the horn. Otherwise, the suspended part of the transducer will incur significant deflection during bonding when a load is experienced at the tip of the transducer.